Recovery of phosphate from phosphate boiler sludge



Oct. 5, 1943. E. w. BUTZLER 2,330,865

RECOVERY OF PHOSPHATE FROM PHOSPHATE BOILER SLUDGE Filed July 24, 1941 .Fecd I Boiler Water Heater 7 5.? E 4 Steam 7 Q U a s= b N f N 5 3 V1 6 TaSewer q i 5 Flash CaS0 "Concenv'atar Sludge 2/2 as Phoqphate Acid .E-eatment azalg oy I I zVa zeah'tq 72 a 10 H; OJgZeaZite N1 ZeaZite N H60 a Ca 612 I Na CZ Na Cl 19 Na S0 M2550 INVEN'I'OR Edward WBuzzler Patented Oct. 5,1943

UNITED STATES PATENT OFFICE amass r mwovaar or momma mom PHOSPHATE BOILER SLUDGE EdwardVEButIlenWestViemPasasslgmr-to Mmmmmh.

Hall orationoilumsylnnia Pans-cor- Application July :4, 1941 semi No. 403,115 8 Claims. (Cl. 210-16) invention relates to a process of recovering phosphate from phosphate boiler sludge. In

materials in solution in the boiler water, which are said to have an efiect upon the so-called caustic embrlttlement oi boiler material.

The use or phosphate radical in various forms has become prevalent in the t of boilerwater for the prevention scale at high-boiler p and the maintenance of alkalinities which are not conducive to caustic embrittlement. By phosphate radical is'meantboththe orthophosphate'radical obtained by solution of soluble alkali-metal salts and hydro en salts of trl-baslc orthophosphoric acid, as well as alkalimetal nmlecularly dehydrated phosphates such as metaphosphates, pyrophosphates and trlpolywhichintlie boilerareunder suitable conditions convertedtoortho mi: 9; A inwhichphosphateradicalisad to the boiler water to prevent the formation of adherentscaleandtocausetheprecipitahon of calcium as a non-adherent sludge is described in Hall Patent 1,613,656, granted January 11, 1927. The cost of phosphat salts for controlling the precipitation or calcium in the term of non-adherent sludge represents an appreciable expense and it is'an objector the present invention to provide a process whereby a large proportionoithephosphateinthesludgemayberecovered for reuse in the boiler water.

- The phosphate usedior water conditioning is iorthe mostpartprecipitatcdintheboilerwater as tricalcic orthophosphate CaaG'OOz, although insomeinstances asubstanual ormfl-ior portion of this precipitation may occur after the addition of the soluble phosphate to the feed water sy tem but before the water reachesthe boiler. Ac-

or other means of accumulation for the tricalcic orthophosphate precipitate are provided, whether such precipitation occurs in the boiler or in the iced water system or is removed by blowdown from the boiler and is settled out from this blowdown water. This tricalcic sludge is removed periodically or continuously in the iorm oi a more or less concentrated sludge and is treated with a sufllcicnt quantity of suitable acid. I! consists for the most part of tricalcic orthophossuch as sulphuric, hydrochloric, nitric, phosphoric, carbonic, sulphurous or acetic, to bring the tricalcic orthophosphate into solution as acid salts of phosphoric acid and calcium, or as phosphorlc acid and calcium salts of the treating acid, or as a mixed system of dissolved calcimn salts and acids.

For example, the addition of sulphuric acid to trlcalcic orthophosphate in suitable concen- 10 trations will bring the same into solution containing hydrogen and calcium ions and sulphate and orthophosphate radicals. The solution resulting irom dissolving the trlcalcic orthophos-' phate sludge in acid is then passed through a base-exchange material, for example sodium zeolite which possesses the property of giving up sodium ion to the solution in exchange for calcium ion. Such basechange materials are of various natural and synthetic, and of various eiliciencies and resistivities to the action of acid solutions. The mixed solution containing phosphate radical is passed through the sodium base-exchange material. whereby the calcimn of the acid solution is taken up by the solid and the sodium of the zeolite is taken into solu-.

tion. For example, tricalcic orthophosphate which has been treated with sulphuric acid to give in solution calcium and hydrogen ions and sulphate and phosphate radicals with portions of so some or all of the un-ionized salts corresponding thereto; will give when passed through a baseexchangematerial a solution substantially free from calciumand containing hydrogen, phosphate, and sulphate ions, and a certain amount of some or all of the un-ionizedsalts This eflluent solution which correspond thereto. .from the base-exchange material is used as such or in the evaporated, crystallized or diluted form as a source of phosphate radical for the treatment and conditioning of feed water for the boiler.

The accompanying drawing is a diagrammatic illustration in flow sheet form of one manner of carrying out my invention.

Referring now more particularly to the according to the present invention. settling basins companying drawing, the boiler water is heated in a feed water heater 2 and delivered from the heater to a boiler 3. Thewat'er delivered to the feed water heater contains phosphate radical from a source which will be described more in the sludge and the steam is returned through a conduit 1 to the feed water heater 2. The'tricalcic orthophosphate sludge contains some entrained soluble phosphate, for example trisodium orthophosphate. The soluble phosphate present in the blowdown water may be allowed to run to waste but preferably it is precipitated by adding thereto calcium sulphate, lime, calcium chloride or some other alkalineearth metal salt which will cause the soluble phosphate to be precipitated as tricalcium phosphate or other insoluble phosphate thus:

Equation 1 3CaSO4+2NaaPO4-+3NazSO4+Ca; (P04) 2 The soluble salts from the blowdown and from the interaction of the added calcium compounds with the soluble phosphate in the blowdown are preferably washed out, in order to keep the total solids in the boiler wateras low as possible.

The tricalcic orthophosphate sludge and the tricalcic phosphate precipitated in the flash concentrator flow from the bottom of the concentrator through a conduit 8 into an acid treating vessel 9. In the vessel 9, acid is added to the sludge to dissolve it. The acid used for dissolving the sludge may be sulphuric, hydrochloric, nitric, phosphoric, carbonic, sulphurous, acetic or any other suitable acid which will bring the sludge into solution. Taking sulphuric acid as the acid used in dissolving the sludge, the reaction may be represented thus:

Equation 2 Ca3(PO4)2+2HzSO4- Ca(HzPO4)z-f-CaSOi Both the calcium sulphate and the monocalcium dihydrogen phosphate resulting from Equation 2 are in solution because of the acidity of the solution and the small concentration of the salts. If the amount of sulphuric acid used Equation 3 Ca: (P04) 2+H:SO4- 2CaHPO4+CaSO4 If, on the other hand, the proportion of sulphuric acid is increased ascompared to the amount used in Equation 2, the acid solution will contain phosphoric acid HzPO4 and calcium sulphate. Corresponding salts in solution will be formed, if other acids are employed in place of sulphuric acid for dissolving the sludge.

The solution resulting from the acid treatment of the sludge is filtered and then passed through the conduit l0 and through a baseexchange material ll containing, for example, sodium or other alkali-metal zeolite. Where the acidity of 'the solution-to be passed through the base-exchange material is not such as to unduly attack the latter, the zeolitic material may be 01' either the inorganic or organic variety. These base-exchange materials are well known per se and therefore are not described in detail. Where, however, the solution to be passed through the base-exchange material is of such high acidity that it would unduly attack an inorganic-zeolite.

the organic type of base-exchange material should be employed.

In passing through the sodium base-exchanger I I, the calcium salts in the acid'solution are converted into sodium salts, the sodium of the base exchanger being exchanged for the calcium oi the solution, so that the eflluent from the baseexchanger contains in solution the ions corresponding to sodium phosphate. Thus if in the acid treatment in the vessel 8 sulphuric acid has been used in amount to produce a solution containing Ca(H2PO4)2 and CaSO4, the reactions occurring in the base-exchanger llmay be represented as follows:

Equation 4 Equation 5 CaSOe-l-Naz base-exchanger- Ca base-exchanger-i-Nazsoi The eiiiuent solution from the base-exchange material II, which solution contains phosphate and sodium ions, is then returned by a pump i2 and conduit H to the feed water heater 2. The soluble phosphate, for example NaHzPO4, either as such or in a more alkaline form, causes the precipitation of calcium in the boiler water as tricalcic orthophosphate sludge, which is then treated as just described.

As a modification of the process which has been described, at least a portion 01 the acid which is to be used in dissolving the sludge in the vessel 9 may be obtained by Passing raw water containing calcium ions and acid radicals through a hydrogen-containing organic baseexchange material, so that the efliuent from this material is an acid solution. For example, consider a raw water containing CaSO4, Ca(HCO:) CaClr, NaCl and NazSOr. A portion of this raw water is passed through a conduit l5 into and through hydrogen-containing organic base-exchange material IS, for example hydrogen organic zeolite. The calcium and sodium of the raw water are exchanged for the hydrogen 0! the base-exchange material [6, so that the emuent from 16 contains HzCOa, H2804 and HCl. Thus:

Equation 6 H: base-exchanger+Ca(HCO:)2-1

Ca base-exchanger+2H:C0:

Equation 7 H: base-exchanger+CaSOr- Ca base-exchanger+HaSO4 Equation 8 H2 base-exchanger+CaClz- I Ca base-exchanger-HHCI Equation 9 Equation 10 NazSO1+Hz base-exchanger Na: base-exchanger+HaB04 concentrator I.

Another portion of the raw water previously referred to is passed through a conduit it into sodium base-exchange material I 9. The calcium sulphate oi the raw water is converted according to Equation into sodium sulphate. The (8002): is converted into NaHCOa according to Equation 11:

Equation 11 Ca-(HCO:):+Naz base-exchanger Ca base-exchanger+2NaHCO:

The calcium chloride is converted into sodium chloride according to Equation 12:

Equation 12 CaCh+Naz baseexchanger Ca base-exchanger+2NaCl NaHsPO-l react to form trlsodium orthophosphate.

Thus:

' Equation 13 NaHzPO4-l-2NaOH Na:P04+ 2E0 (The trisodium orthophosphate reacts with the calcium in the boiler water to precipitate tricalcic orthophosphate according to the following equation:

Equation 14 zsesrol+sca om meuroogsneori 4 Where the feedwater does not contain adequate potential alkali in the form of bicarbonate and carbonat it may be desirable to feed caustic soda or soda ash at some appropriate point between the base-exchanger ii and the boiler 3, preferably directly to the latter, where prior addition might result in feed-line scale of calcium phosphate. Iu'any case, the alkalinity in the boiler is controlled to maintain a desirable low level.

In some cases. phorus-containing material to the system in order to replace the portion which is lost in the various steps or the process. In such case, I may add phosphate rock fltotheac'dtreatingvesseli along with the acid and the sludge. The phosphaterockisdissolvedbytheacidinamanner similar to w to produce a solution containing calcium ions and te radicals. throu the conduit II and the base-exchanger ii and returned to the feed water heater as described. Of course, I could treat the phosphate rock with the acid solution coming from the base-exchange material It in a vessel separate from the vessel 9, in order to solubilize the phosphate rock and then add the solution or the phosphate rock to the solution formed by dissolving the sludge in acid. Furthermore, I could dissolve the phosphate rock in acid which is obtained from some other source thanbypassingraw waterthroughthebaseexchang material it.

It is desirable in carrying the return of boiler. One way oi. accom utilllecarbon dioxide or sulphur dioxide as the dissolving agent for the calcium phosphate sludge.

out the process to lved salts to the III to the it may be desirable to add phosthe dissolvin or the phosphate sludge solved in acid The acid solution is then passed through the base-exchange material It as previously described and the carbon dioxide is stripped irom the sodium phosphate solution in the Ieedwater heater 2. By pulling oil the vapor from the feedwater heater through a condenser, the carbon dioxide can be recycled and used again in the acid treatment vessel 9 where the calcium phosphate sludge is dissolved. Any loss or carbon dioxide may be made up by recovery of some of this constituent from the stack gas 01- the plant. By using sulphur dioxide in place '01 carbon dioxide for dissolving the calcium phosphate sludge, and controlling the stripping operation in the feedwater heater, it is possible to leave enough sulphur dioxide in I Equation: 15 Ca base-exchanger-l-NaCl and sulphur dioxide may be plant stack gases, it-may be Na: base-exchanger+CaCh a similar manner, the zeolite l6 may be regenerated withsulphuric, hydrochloric or other suitable acid according to the equation: Equation 16 Ca base-exchanger+H:SO4

H2 base-exchanger+CaSO4 It will be seen that a cyclic process has been provided whereby phosphate radical acts as a carrier of sodium ion into the feed or boiler water, forces the exchange or this sodium for the calcium in the boiler water according to the above mentioned Hall Patent 1,618,656 and the calcium is precipitated as non-adherent calcium phosphate sludge.

and the acid solution is passed through base-exchange material or the type con- This solution is then passed thisis'to' tainlng sodium whereby the sodium of the baseexchange material is exchanged {or the calcium 0! the solution and a solution is produced con.- taining sodium ions and phosphate radicals which are returned to the boiler. At least a portion of the acid to be used in dissolving the sludge may be obtained by calcium ions and acid radicals through hydrogen containing organic base-exchange material to produce an acid solution.

process, a phosphate material such as phosphate rock may be dissolved in acid and then passed through a sodium-containing organic base-exchanger to produce a solution containing phos phate radicals and sodium ions.

The invention is not limited to the preferred materials or process or apparatus, which have been used merely for illustrative purposes, but the invention may be otherwise embodied or practiced within the scope or the following claims.

I claim:

1. The'prcoess or recovering phosphate from boiler sludge containing phosphate and alkaline arth metal, which comprises dissolving the sludge with acid, and passing the resulting dilute solution through alkali-metal base-exchange mathe feedwater to act as a scavenger for any oxygen present. Since a mixture.

only to regenerate phosphate from Thisphosphate sludge is then dispassing water containing Where necessary in .order to compensate for loss of phosphate in the terial whereby alkaline earth ion present in the solution is replaced by alkali-metal ion.

2. The process of recovering phosphate from boiler sludge containing phosphate and calcium,

which comprises dissolving the sludge with acid,'

'and passing the resulting dilute solution through sodium inorganic base-exchange material whereby calcium ion present in the solution is replaced by sodium ion.

4.-The process of recovering phosphate from boiler sludge containing phosphate and calcium, which comprises dissolving the sludge with acid, passing the resulting dilute solution through sodium base-exchange material whereby calcium ion present in the solution is replaced by sodium ion, and returning the solution containing phosphate radical and sodium ion to the boiler to precipitate calcium from the boiler water in the form of calcium phosphate sludge.

5. A process according to claim 4, in which at least a portion of the acid used in dissolving the sludge is obtained by passing water containing calcium ion and acid radicals through hydrogen containing organic base-exchange material.

6. The process of recovering phosphate from boiler water containing sludge and soluble phosphate, which comprises adding a reagent to the boiler water to form a precipitate from the soluble phosphate, collecting the precipitate and sludge, dissolving the precipitate and sludge with acid, and passing the resulting solution through alkali-metal base exchange material whereby alkaline earth ion present in the solution is replaced by alkali-metal ion.

7. A process according to claim 4, in which alkaline earth phosphate material other than the phosphate sludge is dissolved in acid and the resulting solution is passed through base-exchange material.

8. The process of recovering phosphate from boiler sludge containing phosphate and calcium, which comprises dissolving the sludge with sulphuric acid to form a solution containing calcium, phosphate, and sulphate radicals, passing the resulting dilute solution through sodium base exchange material whereby calcium radical present in the solution is replaced by sodium radical, and returning the solution containing sodium, phosphate and sulphate radicals to the boiler to precipitate calcium from the boiler water in the form of calcium phosphate sludge and to control caustic embrittlement in the boiler.

EDWARD W. BUTZLER. 

